1. Field of the Invention
The subject invention relates to an improvement in control systems for Reaction Wheel Assemblies (RWAs) or Momentum Wheel Assemblies (MWAs) which are variable speed actuators used primarily for stabilizing space vehicles such as satellites.
2. Description of the Prior Art
Reaction Wheel Assemblies (RWs) and Momentum Wheel Assemblies (MWAs) belong to a class of devices generically referred to as momentum-exchange actuators. There are two types of momentun-exchange actuators: the variable speed type which includes RWAs and MWAs and the constant speed type which includes Control Moment Gyroscopes (CMGs). The variable-speed actuator is basically an electrical motor coupled to a flywheel which is referred to as the rotor or sensitive element of the RWA.
Momentum-exchange actuators are used in momentum-exchange systems to control the attitude of space vehicles. In these systems, momentum is transferred between the momentum storage device, i.e., the inertial element of the RWA or CMG, and the vehicle. In a system employing an RWA or MWA, attitude control is produced by changing the apportionment of the total angular momentum of the system which is held constant, between the momentum storage device and the vehicle. The apportionment of this momentum is changed by the control torque which is exerted on the vehicle by the RWA and is proportional to the motor's reaction torque as the rotor is accelerated or decelerated. This control torque is equal to the product of the rotor's inertia and its angular acceleration. The momentum storage capacity of the inertial system is the product of the rotor's inertia and the maximum speed produced by its spin motor. The magnitude of the angular momentum, H, of the RWA is variable and proportional to the angular velocity of the sensitive element while the direction of the angular momentum, H, with respect to the vehicle is fixed.
In prior art RWA control systems a computer processes data received from sensors which monitor the attitude of the vehicle. The computer produces an output signal which is the torque command signal T.sub.C, for application to the sensitive element of the RWA. The torque command signal is coupled through an amplifier and applied to the windings in a torquer motor. The resultant change in current flow in the torquer windings produces a change in the force exerted on the rotor of the motor which is coupled to the shaft of the sensitive element. The resultant torque applied to the sensitive element produces an acceleration or a deceleration of the rotational velocity, .omega., of the sensitive element.
However, the torque produced by the motor, T.sub.M, is adversely affected by disturbance torques, T.sub.B, which adversely affect the motor torque by increasing or decreasing, depending upon the direction of rotation of the sensitive element, the actual torque, T.sub.V, applied to the shaft of the sensitive element. Since the actual torque, T.sub.V, is not equal to the commanded torque, T.sub.C, inaccuracies in the space vehicle attitude are produced. In order to obtain accurate control of the space vehicle, attitude means must be provided to enable the actual torque, T.sub.V, to be substantially equal to the commanded torque, T.sub.C. This requires the provision of means which will compensate for the adverse effects produced by the action of the disturbance torques, T.sub.B, on the motor torque, T.sub.M.
The closed-loop control improvement described herein provides a simple relatively inexpensive means for providing accurate control of the actual torques applied to the sensitive element by compensating for the adverse effects of the disturbance torques, T.sub.B.